Emitter Coupled Logic (ECL) is a family of high-speed digital logic devices which are commonly used in high performance digital systems. Traditionally ECL has been used in mainframe computers, wherein the fastest possible switching rates translated almost directly into system performance. However, today ECL is increasingly being used in the areas of communications, instrumentation and peripherals.
In the design of large digital systems, it is usually desirable to organize data paths into multiply driven busses. This provides a cost savings and size savings by allowing several line drivers to share a common bus. This also saves line receivers, since data is received on only one bus. The prior art ECL busses have traditionally been OR-ed busses constructed using emitter follower outputs of the ECL gates and pull down resistors. These busses have several severe disadvantages, for example: susceptibility to ground loops, noise, and high frequency oscillations. These problems may be avoided by using a differential or balanced bus. Although differential bus receivers are available in integrated ECL circuits, differential bus drivers for multiply driven busses are not available. The differential bus driver function is therefore normally achieved through the use of complementary gates, or single gates with resistor termination. This approach has many of the same problems as the OR-ed bus because, by coupling directly onto the supply rails, noise and other unwanted transients are introduced to the bus, thereby reducing the reliability, increasing the chances of radiating interference, and other problems. Furthermore, the surges of current when the driver changes state are coupled to the supply rails, thereby adding more noise to the system.